1. Field of Invention
The invention relates to microelectromechanical structures (MEMS). In particular, this invention relates to bi-directional thermal actuators made using MEMS technology.
2. Description of Related Art
MEMS devices and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created that are capable of motion or applying force, including, for example, microgears and micromotors. These MEMS devices can be employed in a variety of applications including hydraulic applications in which MEMS pumps or valves are used and optical applications that include MEMS light valves and shutters.
The manipulation of micromachined structures for applications such as microassembly is a delicate task handled best by on-chip devices. One such device, known as a horizontal thermal actuator (HTA), can be used to physically move other parts on or off chip. FIG. 1 shows a conventional HTA 1000. The HTA 1000 is formed on an insulated substrate (not shown), typically a nitride insulated silicon substrate, and includes a first cantilever beam 1120, a first anchor 1130, a second cantilever beam 1150 and a second anchor 1160. The first anchor 1130 anchors the first cantilever beam 1120 to the substrate at a proximal end 1122 of the first cantilever beam 1120. The first cantilever beam 1120 has a distal end 1127. The second anchor 1160 anchors the second cantilever beam 1150 to the substrate at a proximal end 1152 of the second cantilever beam 1150. The second cantilever beam 1150 has a distal end 1157. The first cantilever beam 1120 has a portion 1125 that is wider than the second cantilever beam 1150. The distal end 1127 of the first cantilever beam 1120 is coupled to the distal end 1157 of the second cantilever beam 1150. The first cantilever beam 1120 is electrically connected to ground at the proximal end 1122 of the first cantilever beam 1120 and the second cantilever beam 1150 is electrically connected to a current source 1170 at the proximal end 1152 of the second cantilever beam 1150. The first cantilever beam 1120 is made from the same material as the second cantilever beam 1150.
In operation, current is applied by the current source 1170 to the second cantilever beam 1150. The current applied by the current source 1170 causes the second cantilever beam 1150 to heat up. The first cantilever beam 1120 also heats up very slightly, but only insignificantly. The second cantilever beam 1150, being of smaller width than the first cantilever beam 1120, has a higher current density than the first cantilever beam 1120. The higher current density in the second cantilever beam 1150 causes the temperature of the second cantilever beam 1150 to increase much more rapidly than the first cantilever beam 1120 (thus, beam 1120 is referred to as a xe2x80x9ccold armxe2x80x9d, whereas beam 1150 is referred to as a xe2x80x9chot armxe2x80x9d). Thus, beam 1150 heats up much faster than beam 1120, which, in turn, causes the second cantilever beam 1150 to longitudinally expand relative to beam 1120, and therefore move toward the first cantilever beam 1120. As a result, the coupled ends 1127 and 1157 of the first cantilever beam 1120 and the second cantilever beam 1150 move in the direction of arrow A. When the current supplied by the current source 1170 is removed, the second cantilever beam 1150 quickly cools and returns to its original position, unless another object prevents it from moving back.
The conventional HTA is reliable and typically requires less than 5 volts, making it CMOS compatible. However, it can exert force in only one direction. In many applications, it is desirable to exert force in two opposite directions. In order to exert force in opposite directions, one could provide a first set of one or more HTAs that operate in a first direction, and a second set of one or more HTAs that operate in a second, opposite direction. However, this doubles the overall size and amount of material required. In addition, bi-directional actuators have been developed that use two different materials and multiple layers having different coefficients of thermal expansion, but such devices have exhibited bending in the off state. Further, the manufacturing process for these devices is cumbersome because of the need to accommodate two different types of materials.
One aspect of this invention provides a thermal actuator that can apply force to an object in two directions without using two different materials having different coefficients of thermal expansion.
Another aspect of this invention provides a bi-directional thermal actuator which can be incorporated into an array of such actuators to apply force in two directions without sacrificing valuable chip space.
Another aspect of this invention provides a bi-directional actuator that is easily manufactured.
The bi-directional actuator includes first and second xe2x80x9chotxe2x80x9d arms (instead of a single xe2x80x9chotxe2x80x9d arm), and a third arm, which is the xe2x80x9ccoldxe2x80x9d arm. The bi-directional actuator preferably is made of a single material, for example, polysilicon, using a MEMS process such as, for example, surface micromachining.
According to one embodiment, a bi-directional actuator includes first, second and third arms, each being parallel to each other, and each having distal and proximal ends. The third arm is arranged between the first and second arms. The third arm has at least one portion between its proximal and distal ends with an in-plane width that is wider than an in-plane width of each of the first and second arms. The distal ends of the first, second and third arms are coupled together to form a distal end of the bi-directional thermal actuator. The proximal end of the third arm can be connected to ground, whereas the proximal ends of the first and second arms selectively have current applied thereto. The first, second and third arms preferably are made of the same material. When current is applied to the first arm, the distal end of the thermal actuator moves and applies force in a first direction. When current is applied to the second arm, the distal end of the thermal actuator moves and applies force in a direction opposite to the first direction.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.